The Wnt signaling pathway provides a paradigm for how misregulation of cell signaling contributes to oncogenesis. Loss-of-function mutations in the tumor suppressor protein, Adenomatous polyposis coli (APC), inappropriately activate Wnt signaling and frequently initiate cancer progression. In fact, mutations in APC are responsible for over 80% of all colon cancer cases, thus it is clear that understanding APC biology could provide important insight to develop novel therapeutic strategies. In the current model of Wnt signaling, APC participates in a multi-protein ?destruction complex? that targets the key effector protein ?-catenin for ubiquitin-mediated proteolysis. In the absence of APC, the destruction complex is inactivated, ?-catenin hyper-accumulates, and it translocates to the nucleus where pro-proliferative genes such as c-myc and cyclinD1 are transactivated. A long-term goal of my lab is to understand how ?-catenin signaling is regulated, and we are currently focused on understanding how the destruction complex works as a molecular machine. While it is clear that APC participates in the complex, its mechanistic role remains unknown. In the past decade, several models have been proposed to explain APC's mechanistic function in the destruction complex. While these models are based on convincing biochemical data, they remain to be rigorously tested using functional studies. Drosophila APC2 is an ideal model for such functional studies, as Wnt pathway components are highly conserved across metazoans, and I have previously shown that Drosophila APC2 works as effectively as human APC in human colon cancer cell lines. My lab therefore uses a multi-disciplinary approach that combines state-of-the-art cell biological and biochemical tools in human cell culture, with the powerful genetic in vivo system of Drosophila development to investigate questions important to the progression and treatment of human colon cancer. During our prior funding cycle, my lab utilized a structure/function approach to investigate the importance of different ?-catenin binding sites on APC for proper Wnt regulation. Our findings are inconsistent with existing models, and instead suggest an alternative model of the inner workings of the destruction complex. Here, I propose experiments to directly evaluate this model, and to pursue studies investigating how APC can modulate Wnt signaling outputs independent of ?- catenin destruction.